The present invention relates to impulse transmission systems and, more particularly, to a method and apparatus for generating a pulse train having specifiable spectral response characteristics for use by an impulse transmission system.
As the availability of communication bandwidth in the increasingly crowded frequency spectrum is becoming a scarce and valuable commodity, Time Modulated Ultra Wideband (TM-UWB) technology provides an excellent alternative for offering significant communication bandwidth, particularly, for various wireless communications applications. Because TM-UWB communication systems are based on communicating extremely short-duration pulses (e.g., pico-seconds in duration), such systems are also known as impulse radio systems. Impulse radio systems were first described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990), and U.S. Pat. No. 5,363,057 (issued Nov. 8, 1994) to Larry W. Fullerton, and U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997), and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Larry W. Fullerton, et al. These patents are incorporated herein by reference.
Multiple access impulse radio systems are radically different from conventional Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) systems. Unlike such systems, which use continuous sinusoidal waveforms for transmitting information, a conventional impulse radio transmitter emits a low power electromagnetic train of short pulses, which are shaped to approach a Gaussian monocycle. As a result, the impulse radio transmitter uses very little power to generate noise-like communication signals for use in multiple-access communications, radar and positioning applications, among other things. In the multi-access communication applications, the impulse radio systems depend, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high achievable processing gains, the impulse radio systems are relatively immune to unwanted signals and interference, which limit the performance of systems that use continuous sinusoidal waveforms. The high processing gains of the impulse radio systems also provide much higher dynamic ranges than those commonly achieved by the processing gains of other known spread-spectrum systems.
Impulse radio communication systems transmit and receive the pulses at precisely controlled time intervals, in accordance with a time-hopping code. As such, the time-hopping code defines a communication channel that can be considered as a unidirectional data path for communicating information at high speed. In order to communicate the information over such channels, typical impulse radio transmitters use position modulation, which is a form of time modulation, to position the pulses in time, based on instantaneous samples of a modulating information signal. The modulating information signal may for example be a multi-state information signal, such as a binary signal. Under this arrangement, a modulator varies relative positions of a plurality of pulses on a pulse-by-pulse basis, in accordance with the modulating information signal and a specific time-hopping code that defines the communication channel.
In applications where the modulating information signal is a binary information signal, each binary state may modulate the time position of more than one pulse to generate a modulated, coded timing signal that comprises a train of identically shaped pulses that represent a single data bit. The impulse transmitter applies the generated pulses to a specified transmission medium, via a coupler, such as an antenna, which electromagnetically radiates the pulses for reception by an impulse radio receiver. The impulse radio receiver typically includes a single direct conversion stage. Using a correlator, the conversion stage coherently converts the received pulses to a baseband signal, based on a priori knowledge of the time-hopping code. Because of the correlation properties of the selected time-hopping codes, the correlator integrates the desired received pulses coherently, while the undesired noise signals are integrated non-coherently such that by comparing the coherent and on-coherent integration results, the impulse receiver can recover the communicated information.
Conventional spread-spectrum code division multiple access (SS-CDMA) techniques accommodate multiple users by permitting them to use the same frequency bandwidth at the same time. Direct sequence CDMA systems employ pseudo-noise (PN) codewords generated at a transmitter to xe2x80x9cspreadxe2x80x9d the bandwidth occupied by transmitted data beyond the minimum required by the data. The conventional SS-CDMA systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence synchronized to the family of codes. Each user is assigned one of the spreading codes as a spreading function. One such spread-spectrum system is described in U.S. Pat. No. 4,901,307 entitled xe2x80x9cSPREAD-SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d by Gilhousen et al.
Unlike direct sequence spread-spectrum systems, impulse radio communications systems have not employed time-hopping codes for energy spreading, because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the impulse radio systems use the time-hoping codes for channelization, energy smoothing in the frequency domain, and interference suppression. The time-hoping code defines a relative position of each pulse within a group of pulses, or pulse train, such that the combination of pulse positions defines the communications channel. In order to convey information on such communication channel, each state of a multi-state information signal varies a relative pulse position by a predefined time shift such that a modulated, coded timing signal is generated comprising a train of pulses, each with timing corresponding to the combination of the time position coding and the multi-state modulation.
In one conventional binary modulation approach, pulses are time-modulated forward or backward about a nominal position. More specifically, each pulse is time modulated by adjusting its position within a time frame to one of two or more possible times. For example, in order to send a xe2x80x9c0xe2x80x9d binary bit during the time frame, the pulse may be offset from a nominal position of the time frame by about xe2x88x9250 picoseconds. For a xe2x80x9c1xe2x80x9d binary state, the pulse may be offset from the nominal position by about +50 picoseconds. Conventional coders that generate the time-hoping code do so in response to a periodic timing signal that corresponds to the data-rate of the multi-state information signal. The data rate of the impulse radio transmission may for example be a fraction of a periodic timing signal that is used as a time base or time reference.
In practice, decoding errors are minimized using distinctive time-hopping codes with suitable autocorrelation and cross-correlation properties. The cross-correlation between any two time-hopping codes should be low for minimal interference between multiple users in a communications system or between multiple target reflections in radar and positioning applications. At the same time, the autocorrelation property of a time-hoping code should be steeply peaked, with small side-lobes. Maximally peaked time-hopping code autocorrelation yields optimal acquisition and synchronization properties for communications, radar and positioning applications.
Various coding schemes with known correlation characteristics are available. For example, algebraic codes, Quadratic Congruential (QC) codes, Hyperbolic Congruential (HC) codes and optical codes have been suggested in the past for coding in impulse radio systems. Generally, based on known assumptions, the coding schemes guarantee a maximum number of pulse coincidences, i.e., hits, for any defined time frame or time frame shift during which the codes are repeated. For example, HC codes are guaranteed a maximum of two hits for any subframe or frame shift.
McCorkle in U.S. Pat. No. 5,847,677 discloses a random number generator for generating a pseudorandom code for use with jittered pulse repetition interval radar systems. The code is generated by a random number generator that possesses certain attributes deemed desirable for a jittered radar. As disclosed, the claimed attributes related to a flat frequency spectrum, a nearly perfect spike for an autocorrelation function, a controllable absolute minimum and maximum interval, long sequences that do not repeat, and a reasonable average pulse rate.
One known coding technique for an impulse radio is disclosed by Barrett in U.S. Pat. No. 5,610,907, entitled xe2x80x9cUltrafast Time Hopping CDMA-RF Communications: Code-As-Carrier, Multichannel Operation, High data Rate Operation and Data Rate on Demand.xe2x80x9d According to the disclosed techniques, two levels of coding are used: major orthogonal codes are applied to provide multiple channels, and forward error correction (FEC) codes are applied to information data before transmission. The disclosed system relies on dividing time into repetitive super-frames, frames and subframes. As disclosed, a super-frame corresponds to a time interval of about 1 millisecond, representing one repetition of a code pattern, where as a frame is defined as a time interval of about 1 microsecond divided according to a code length. A subframe corresponds to a short time interval of about 1 nanosecond during which a pulse is time positioned.
Because of practical limitations associated with arbitrarily positioning of pulses in adjacent frames, each frame may have to be divided into allowable and non-allowable time regions for positioning a pulse. One such limitation is associated with hardware limitation on minimum pulse-to-pulse time for respective positioning of two pulses on adjacent frames arbitrarily. The system disclosed in Barrett uses a fraction of frame time for encoding and designates the remainder as a RESET period.
Time-hopping coding efforts have primarily focused on correlation properties and only limited efforts have been applied towards using coding techniques to affect the spectral properties of pulse trains. Designed code generation techniques such as those disclosed by Barrett are employed solely due to the desirable correlation properties of the produced codes. Indeed, the spectral properties of individual codes within families of such codes vary tremendously from code to code. Although pseudorandom codes such as those disclosed by McCorkle can be used to produce pulse trains with relatively flat spectrums, the frequencies at which peaks and valleys occur within the energy spectrum vary significantly with each pseudorandom sequence.
Methods have been disclosed that combine pseudorandom coding and other pulse placement techniques to achieve notches in the energy spectrum of pulse trains. One approach is disclosed in U.S. Pat. No. 5,748,891 entitled xe2x80x9cSpread Spectrum Localizersxe2x80x9d by Fleming, et al. The approach described by Fleming uses a pseudorandom code to specify the positions of pulses that are each paired with an inverted pulse that is deterministically positioned such that a notch in the energy spectrum is produced at a desired frequency. However, such methods are significantly limited in their ability to otherwise shape the energy spectrum of a pulse train.
Because TM-UWB technology is applicable to a wide variety of applications including communications, radar, and positioning, different forms of pulse trains are needed that have varying spectral and correlation properties. Numerous methods for producing pulse trains with desirable correlation properties are available. But, few techniques exist for shaping the energy spectrum of pulse trains to meet spectral property requirements. Therefore, improved methods for shaping the energy spectrums of pulse trains are needed.
Generally described, the present invention optimizes the spectral properties of a pulse train for use by an impulse transmission system. A system and method generates at least one code specifying an initial pulse train having certain spectral properties and correlation properties, and modifies the temporal and/or non-temporal characteristics of pulses in the pulse train until acceptance criteria are met or xe2x80x98best fitxe2x80x99 is attained. Various methods for specifying characteristic value layouts are described including value range layouts, discrete value layouts, and combined value range-discrete value layouts, each of which can be employed using fixed or non-fixed values. Several code mapping approaches are described that can be used to map code elements to one or more values in one or more characteristic value layouts. Various approaches are described for modifying the characteristics of pulses within a pulse train. Methods are provided for determining the spectral properties of a pulse train and a weighting curve approach to assist in spectral shaping is described.
Pulse characteristics that can be modified include pulse position (in time), amplitude, width (in time), type, and polarity. Pulses can also be added to and deleted from a pulse train. Approaches for searching for optimal combinations of pulse characteristic values are described including single-sample and multiple-sample modify-and-compare methods and repeating single-sample and multiple-sample modify-and-compare methods. Single-pass and multiple-pass approaches to varying the order of varying characteristic values are also explained.